THE  PROPERTIES  AND  ELECTRODE 
BEHAVIOR  OF  MANGANESE 
AMALGAM 

BY 

WARREN  COBINE  BRUCE 


THESIS 

FOR  THE 

DEGR  E E O E BACHELOR  O F S C I E N G E 

IN 

CHEMICAL  ENGINEERING 


COLLEGE  OF  LIBERAL  ARTS  AND  SCIENCES 

UNIVERSITY  OF  ILLINOIS 


]!)22 


Digitized  by  the  Internet  Archive 
in  2016 


https://archive.org/details/propertieselectrOObruc 


/922 

Be  3 


UNIVERSITY  OF  ILLINOIS 

i9i 

THIS  IS  TO  CERTIFY  THAT  THE  THESIS  PREPARED  UNDER  MY  SUPERVISION  BY 

Warren  Cobine  Eruce 

ENTITLED.. .. 

Amalgam 

IS  APPROVED  BY  ME  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR  THE 
DEGREE  OF  Lachelor_cf .. Science .T.n  .Che^caT.^  


ACKNOWLEDGMENT 


The  author  wishes  to  express  his  apprecia- 
tion of  the  guidance  and  help  given  by  Dr,  John  H. 
Reedy,  under  whose  supervision  this  v/ork  was  car- 
ried on. 


I. 

II. 

III. 


IV. 


TABLE  OP  CONTENTS 


Page 

1 

1 

4 

4 

6 

8 


ACKNOWLEDGMENT 
INTRODUCTION 
HISTORICAL 
EXPERIMENTAL 

A.  Preparation  of  Manganese  Amalgam 

B.  Properties  of  Manganese  Amalgam 
Table  I.  Rate  of  Oxidation  of  Manganese  in 

Manganese  Amalgam 

C.  Electrode  Potential  of  Manganese  Amalgam  9 

and  the  Manganese  Amalgam-Manganese  Di- 
oxide Electrode 

1.  Theoretical  9 

2.  Experimental  10 

Table  II.  Electrode  Potential  Measure-  11 

ments  of  Manganese  Amalgam 

Graph  I.  Electrode  Potentials  oP  Man-  12 

ganese  Amalgams  in  Contact  with  Vari- 
ous Concentrations  of  Manganous  Sul- 
phate 

Graph  II.  Electrode  Potentials  of  Man-  15 
ganese  Amalgams  in  Contact  with  0.500 
Molal  Manganous  Sulphate 

SUMMARY  14 

17 


BIBLIOGRAPHY 


1 


I 

INTRODUCTION 


The  purpose  of  this  work  is  to  investigate  the  properties  and 
electrode  behavior  of  manganese  arid  manganese  amalgam  with  regard  to 
their  use  in  the  development  of  a manganese  amalgam-manganese  dioxide 
electrode  for  the  measurement  of  oxidizing  potentials  of  solutions* 

II 

HISTORICAL 

The  earliest  article  on  manganese  amalgam  was  by  Bftttger1.  Bun- 
sen2 obtained  metallic  manganese  by  electrolyzing  a saturated  solutior 

o 

of  manganous  chloride  at  100  C.  using  platinum  electrodes.  If  the 
current  is  less  than  0.067  amperes  mangano -manganic  oxide  is  obtained 
instead  of  metallic  manganese.  The  metal  comes  down  in  shining 
leaves  which  oxidize  in  air  as  fast  as  potassium.  Moissan3  obtained 
a needle-like,  crystalline  amalgam  of  manganese  by  electrolyzing  a 
concentrated  solution  of  manganous  chloride  using  a mercury  cathode, 
Moore"  found  that  a very  large  proportion  of  the  manganese  in  a solu- 
tion may  be  deposited  as  the  metal,  at  the  cathode,  by  employing  a 
neutral  solution  containing  a large  excess  of  ammonium  thiocyanate. 

A.  powerful  current  is  necessary.  Smith  and  Frankel5  using  an  excess 
of  potassium  sulphocyanide  in  a solution  of  manganous  sulphate  or 
citrate,  upon  electrolysis,  obtained  a grayish- white,  compact  deposit 
of  metallic  manganese  upon  the  cathode,  no  manganese  dioxide  separa- 
ting at  the  anode.  Warwick6  has  suggested  the  use  of  acetate  in  this 


. 


• ‘ 


' 


. 


. 


2 

connection  and  Neumann7  says  that  the  same  deposit  can  be  obtained 
from  neutral  salt  solutions  or  solutions  containing  a small  excess  of 
nitric  acid,  the  acid  in  the  latter  being  converted  into  ammonia  by 
the  action  of  the  current.  The  deposit  of  metallic  manganese  decom- 
poses water.  Guntz8  obtained  an  amalgam  rich  in  manganese  when  he 
electrolyzed  a concentrated  solution  of  manganous  chloride  at  15-20 
amperes  using  400-500  grams  of  mercury  as  the  cathode.  After  strong 
pressing  in  chamois  leather  he  obtained  a solid  amalgam  which  in  air 

remained  unchanged  even  for  a month.  It  is  slate-gray,  begins  to  give 

o 

ip  mercury  at  130  C.  and  decomposes  water  at  ordinary  temperature. 
Prelinger9  prepared  manganese  amalgam  by  electrolyzing  a saturated 
solution  of  manganous  chloride  at  11  volts,  the  temperature  rising  to 

o 

70  C.  After  filtration  thru  linen  and  subjection  to  a.  pressure  of 

2000  kilos  per  sq.  cm.  for  several  hours,  an  analysis  of  the  amalgam 

o 

gave  9.9  /o  manganese,  which  indicates  the  formation  of  the  compound 
VTn2Hg5 . This  compound  is  slate-gray  in  color,  assumes  a metallic  lus- 
ter when  rubbed  or  cut,  and  oxidizes  slowly  in  air  at  ordinary  tem- 

o 

peratures,  metallic  mercury  being  liberated.  At  100-110  G.  it  decom- 
poses into  its  elements.  The  specific  gravity  12.828,  is  a greater 
cumber  than  that  of  12.532  calculated  from  the  specific  gravity  of  its 
constituents,  so  that  contraction  takes  place  in  the  formation  of  the 
compound  Mn2Hg5 . Mn2Hg6  is  electropositive  to  mercury.  Consequently 
leat  is  probably  absorbed  in  its  formation.  A solution  of  the  com- 
pound in  mercury  is  not  attacked  by  dry  air  but  is  quickly  oxidized 
by  moist  air  to  manganic  oxide,  Mn203,  which  forms  a fine  dust  on 
the  surface  of  the  liquid.  Erdman  noted  that  manganese  amalgam 
crystallizes  in  needles.  He  obtained  a semisolid  amalgam  by  the  ac- 


3 


tion  of  sodium  amalgam  upon  a concentrated  solution  of  manganous 
chloride,  as  did  many  other  workers.  Foerster11  states  it  is  possi- 
ble to  deposit  metallic  manganese  at  the  cathode  provided  high  cur- 
rent densities  and  strong,  nearly  neutral,  solutions  are  used.  Ram- 
say12 says  that  manganese  amalgam  rapidly  turns  black  in  air  but  may 
be  preserved  some  time  under  water,  in  a closed  vessel,  on  which  it 
has  little  or  no  action.  Under  alcohol  in  an  open  vessel  it  rapidly 
becomes  oxidized  owing  to  absorption  of  oxygen  from  the  air.  Laden- 
burg13  finds  that  manganese  amalgam  slowly  decomposes  water  with  the 
evolution  of  hydrogen.  Shaking  the  amalgam  v/ith  water  in  contact 
with  air  gives  ozone.  Abegg1-  in  1913  states  that  very  little  is 
known  about  the  electromotive  behavior  of  manganese.  Manganese  is  one 
of  the  more  reactive  metals  and  possesses  a great  affinity  to  form  all 
of  the  metalloids,  and  has  a rather  high  electrolytic  solution  pres- 
sure. Neumann15  found  that  metallic  manganese  gives  unsteady  poten- 
tial values.  He  obtained  better  results  with  electrolytically  pre- 
pared semisolid  manganese  amalgam.  The  values  for  the  amalgam  are 
nore  positive  than  those  of  the  pure  metal.  His  results  are  given 
later.  According  to  Ramsay12,  manganese  is  one  of  the  metals  which 
gives  a normal  lowering  of  the  vapor  pressure  of  mercury.  Van  Ars- 

lale  and  Maier16  found  that  metallic  manganese  can  be  deposited  at  the 

o 

iathode  from  neutral  sulfate  solutions  v/ith  80-90  /0  efficiency  at 
3 volts  or  more,  but  the  deposit  is  powdery  and  a concentration  of 

o 

.35  /o  free  sulphuric  acid  prevents  deposition.  Lorenz  and  Hostelet17 
md  Muthmann  and  Fraunbergens  give  some  potential  measurements  of 
netallic  manganese. 


4 


III 

EXPERIMENTAL 

A.  PREPARATION  OP  MANGANESE  AMALGAM 
Attempts  to  amalgamate  manganese,  prepared  by  the  thermit  process, 
by  direct  contact  with  mercury,  failed*  A piece  of  this  kind  of  man- 
ganese was  washed  with  ether,  dried,  weighed,  and  placed  in  contact 
with  mercury  at  room  temperature  for  three  days.  It  failed  to  show 
any  signs  of  amalgamation,  its  weight  still  being  1.7549  grams.  Mer- 
cury does  not  wet  the  surface  of  this  kind  of  manganese.  On  leaving 
it  in  contact  with  mercury  at  the  temperature  of  the  steam  bath  for 
24  hours  the  manganese  showed  no  signs  of  amalgamation,  but  lost  its 
luster  and  gained  .0005  grams  in  weight  due  to  superficial  oxidation 
to  MnO ( OH ) 2 • 

The  amalgams  used  in  the  experimental  work  were  prepared  by  elec- 
trolyzing concentrated  solutions  of  manganous  sulphate.  The  appara- 
tus consisted  of  a 500  cc.  beaker,  a small  crucible  to  hold  the  mer- 
cury cathode,  a platinum  foil  anode,  and  an  ordinary  extraction  cell 
of  filter  paper  to  cover  the  anode  to  prevent  dispersion  of  manganese 
dioxide  thru  the  solution.  Connection  to  the  mercury  cathode  was 
made  by  means  of  a platinum  wire  fused  in  the  end  of  a piece  of  glass 
tubing  filled  with  mercury.  The  voltages  used  ranged  from  the  theo- 
retical amount  up  to  110  volts.  At  110  volts  a solid  amalgam  is  ob- 
tained in  several  minutes.  The  electrolysis  goes  better  in  hot  solu- 
tions, but  the  solution  need  not  be  heated  externally  as  it  soon  be- 
comes heated  by  the  current.  The  electrolysis  goes  faster  if  the 
solution  is  stirred  but  stirring  was  not  necessary  in  this  work.  No 


5 


reagents  to  prevent  the  deposition  of  manganese  dioxide  at  the  anode 
or  the  solution  building  up  in  acid,  were  used,  for  fear  of  contami- 
nating the  amalgam.  In  this  connection  ammonium  acetate  and  arrmonium 
sulphate  were  tried,  as  they  seemed  to  be  the  least  objectionable.  It 
/ms  found,  however,  that  when  ammonium  salts  are  present,  ammonium 
amalgam  readily  forms.  Ammonium  amalgam  is  apparently  easily  decom- 
posed by  triturating  with  water.  It  is  not  sure,  hov/ever,  whether 
the  last  trace  of  ammonia  can  be  readily  removed,  especially  since 
there  is  a possibility  of  c cmpound  formation  in  the  system  manganese, 
ammonia,  mercury.  Therefore,  ammonium  acetate  or  ammonium  sulphate 
were  not  used.  The  solution,  of  course,  soon  builds  up  in  acid  dur- 
ing the  electrolysis  but  this  has  no  deleterious  effect.  Manganous 
chloride  solution  is  not  as  satisfactory  as  manganous  sulphate  be- 
cause the  evolved  chlorine  corrodes  the  anode.  The  amount  of  manga- 
nese in  'the  amalgams  v/as  obtained  by  weighing  the  mercury  cathode  be- 
fore and  after  electrolysis.  After  electrolysis  the  cathode  vessel 
with  the  amalgam  w as  washed  v/ith  water,  heated  on  the  steam  bath  for 
five  to  ten  minutes , cooled  in  a desiccator  and  v/eighed.  Several 
blank  runs  proved  that  mercury  heated  on  the  steam  bath  for  ten  min- 
utes or  less  could  be  v/eighed  to  the  fourth  decimal  place  v/ithout 
showing  any  loss.  The  amalgam  cannot  be  washed  with  alcohol  v/ithout 
causing  a slight  oxidation  of  the  manganese.  Ether  forms  black  com- 
pounds with  the  mercury  because  ether  contains  sulphur  derivatives 
which  are  difficult  to  remove  completely. 

The  mercury  used  in  the  experimental  v/ork  was  washed  three  times 
v/ith  nitric  acid  by  passing  it  three  times  thru  a Meyer  column,  dried 
and  distilled  in  a pyrex  glass  still  en  vacuo.  The  manganous  sul- 


. 

* ' ‘ " 


. 


. 


. 


■ ■ 


6 


phate  was  prepared  recrystallizing  twice  from  C.  P.  manganous  sul- 
phate. The  other  reagents  used  were  the  ordinary  C.  P.  materials. 

B.  PROPERTIES  OF  MANGANESE  AMALGAM 

The  amalgam  prepared  in  the  manner  described  is  always  hetero- 
geneous. It  is  composed  of  liquid  amalgam, semi so lid  amalgam,  and 
rather  compact  lumps  of  solid  amalgam  which  apparently  have  a crys- 
talline structure  interspersed  with  liquid  amalgam.  Liquid  amalgam 
can  easily  be  forced  from  the  pores  of  the  solid  amalgam  by  merely 
squeezing  a piece  of  the  latter  between  the  fingers.  Even  after 
standing  three  days  the  lumpy  amalgams  did  not  become  homogeneous.  If 
the  amalgams  were  merely  liquid  amalgams  in  equilibriun  with  excess 
manganese  they  probably  would  have  been  entirely  semisolid  rather  thar 
lumpy.  The  formation  of  these  lumps  is  probably  due  to  the  crystal- 
lizing out  of  some  compound  of  manganese  and  mercury,  some  liquid 
amalgam  being  brought  down  with  the  crystals  of  the  compound.  This 
compound,  according  to  Prelinger9'  is  probably  Mn2Hg5 . During  the 
electrolysis  bright  needle  or  leaf-like  crystals  of  am&lgam  form,  on 
the  surface  of  the  mercury  cathode  and  upon  long  electrolysis  these 
crystals  tree  up  and  creep  over  the  edge  of  the  crucible.  This  again 
seems  to  verify  Prelinger’s  conclusion  that  a compound  is  formed  be- 
tween manganese  and  mercury. 

The  lowest  amount  of  manganese  found  in  the  semi sol id  amalgams 

o 

was  0.465  /° , and  it  is  concluded  that  the  solubility  of  manganese  in 
nercury  at  room  temperature  is  not  greater  than  this  amount. 


. 

J 

• 

. 

7 


Trie  approximate  melting  point  of  a dilute  amalgam  (one  volume 

saturated  amalgam  plus  fifty  volumes  mercury)  was  determined  as  being 

o 

about  -35  C.,  by  freezing  the  amalgam  with  liquid  air  and  allowing  it 

to  melt.  This  is  over  three  degrees  higher  than  that  of  mercury 

o 

(-38.85  C.)  and  indicates  that  manganese  and  mercury  form  a compound. 
This  is  another  proof  of  Pre linger !s  conclusion  that  manganese  and 
mercury  form  a compound. 

Manganese  amalgams  of  different  concentration  did  not  all  appear 
to  have  the  same  reactivity  towards  oxygen  so  the  following  ex- 
periments were  carried  cut.  The  saturated  amalgam  was  obtained  by 
filtering  semisolid  amalgam  thru  a filter  paper  containing  pinholes. 
It  probably  contained  a small  amount  of  the  solid  phase.  The  experi- 
mental method  and  results  are  given  in  the  following  table* 


Table  I_. 

Relative  Rates  of  Oxidation  of  Manganese  Amalgams 
of  Various  Concentrations . 


A.  Exposed  to  laboratory  air.  Amalgam  dry. 


Dilute  Liquid  Amalgam 

oat.  Amalgam 
plus  53- 
Volumes  Eg 

Sat.  Amalgam 
plus  1 
Volume  Eg 

Saturated 

Acnalgam 

Semi  solid 
Amalgam 

Solid 

Amalgam 

Instantly . 

Instantly . 

Fast . 

Very  slow. 

Brown  layer 
of  oxide 
which  turns 
black 

Brown  layer 
of  oxide 
’which  turns 
black 

brown  layer 
of  oxide 
in  5 
minutes 

Thin 

brown  layer 
after 
94  hours 

No  signs  of 
oxidation 
ev  en  after 
7)2  days 

a.  Under  water  exposed  to  laboratory 

air. 

Fast  • 

Fast . 

Slow . 

Slow . 

Very  slow. 

Brown 

precipi tate 
after 
several 
minutes . 

Brown 

precipi tate 
after 
several 

minutes 

Black 

layer 

after 

24 

hours 

Well 

developed 
black  layer 
after 
24  hours 

Thi  n 

brown  layer 
after 
24 

hours 

C.  Under  boiled  water  in  a glass-stoppered  bottle  sealed  with 
sealing  wax.  Bottle  filled  as  full  as  possible  with  water 
while  boiling  hot. 


ho  immediate  action  was  noticed  in  any  case 


Small 

Small 

Brown 

amount 

amount 

splotches 

LnO (OH ) o 

MnO  (OH ) r> 

after 

after  72 

after  72 

59 

days 

days 

days 

D.  A piece  of  solid  amalgam  was  partly  exposed  to  water  and 
partly  to  laboratory  air.  The  part  above  water  showed  no 
signs  of  oxidation,  even  after  24  hours.  The  part  below 
water  developed  a brownish  tinge  after  an  hour  and  a well- 
developed  brown  layer  after  24  hours. 

In  all  cases  in  the  experiments  with  with  amalgams  exposed  to  the 
air  the  film  of  oxide  appeared  to  prevent  further  oxidation.  In 
no  case  was  any  evolution  of  gas  noted. 


9 


Some  semis olid  amalgam,  in  a crucible  heated  very  gently  over  a 
bunsen  flame  for  48  hours  still  retained  its  silver-bright  surface 
and  showed  no  signs  of  oxidation. 

Attempts  to  preserve  manganese  amalgam  under  benzene  and  toluene 
failed.  In  both  cases  a film  of  brown  oxide  formed  and  also  a black 
precipitate  due  to  the  action  of  impurities  such  as  sulfur  deriva- 
tives o 

Attempts  to  preserve  manganese  amalgam  under  manganous  sulphate 
and  chloride  solutions  failed.  The  oxidation  is  not  as  rapid  as  in 
the  case  of  water,  probably  because  oxygen  is  not  as  soluble  in  these 
solutions  or  because  the  manganese  in  the  solution  reacts  with  the 

oxygen. 


C.  THE  ELECTRODE  POTENTIAL  OP  MANGANESE  AMALGAM,  AND 
THE  MANGANESE  AMALGAM - MAN GANE SE  DIOXIDE  ELECTRODE 


1 , The ory  of  the  Manganese  Amalgam-Manganese  Dioxide  Electrode . 

It  would  appear  that  in  the  various  forms  in  which  this  electrode 
might  be  developed,  might  be  found  a suitable  combination  for  measur- 
ing the  ©xidi  zing  potential  of  a number  of  solutions.  This  electrode 
should  be  reversible  with  respect  to  manganous  ions. 


The  half  cell  is  of.  the  type  Mn 
In  the  case  of  the  half  cell 
Mn  | MnOa  | MnO^“ 
the  reaction  should  be 

Mn  + © + MnO*"  ^ 


MnOa  0 


2 Mn02 


10 


2 o Experimental . 

E.  M.  F.  measurements  of  the  following  combinations  were  made 
with  a Leeds  andNorthrup  student’s  potentiometer,  a standard  cadmium 
cell,  a tenth  normal  calomel  half  cell,  and  a three  normal  potassium 
chloride  intermediate  solution. 


. . . 


. 


Table  II 


11 


Elec  trode  Potential  Measurements  of  Manganese  Amalfi  ms « 


Referred  to  the  0.1  N calomel  electrode  as  +.613  volts.  The  last 
figure  in  each  case  is  doubtful.  Room  temperature  = 23.5  - 1°  G. 


Saturated 

Saturated 

Saturated 

Amalgam 

Semi  solid 

Hg 

Amalgam 

Amalgam 

Amalgam 

+ 51  vol s 

+ 9 vols 

.4645$  Mn 

Kg 

Hg 

vol  ts 

volts 

volts 

volts 

volts 

.0004-5  MnS04 

+ .765 

- .717 

-.857 

- . 777 

« o M ivuibu^ 

+ .7o4 

-.739 

-.302 

-.800 

- .800 

1.0  M MnS04 
M ilO  p — I • 0 M 

+ .669 

-.859 

-.313 

-.808 

-.839 

MnC  1 2 

-.374 

MnO p—1.0  M 
MnS04 
.00045  M 

-.895 

KMn04 

-.617 

-.957 

MnOp- .00045 
M Kiin04. 

M nG  p—  0.4  M 

-.397 

-1.047 

KMn04 
,000325  M 

-.931 

1.116 

Klin04  in 
.005  it  KCE 

-.857 

.000335  M 

KMn04  in 
.05  M HoS04 

-.743 

MnOp- .000335 

M Kl£n04  in 
.005  M K0H 

-.937 

- *980 

MnO  p- .000235 

M KMn04  in 
.05  JS  HpS04 

-.817 

MnO 0- . 2M 

KMnC4  in 
.005  M KGH 

.939 

.863 

MnO p- . 2 M 

KMn04  in 
.05  N Hp  J4 

1.435 

.973 

In  the  half  cell,  semi  solid  amalgam  -MnO g- 1.0  M MnCl0,  a volume 
of  mercury,  equal  to  that  of  the  amalgam,  was  added  and  the  cell 
shaken,  without  causing  any  change  in  th  e electrode  potential. 


Comparison  Wi  th  Neumann1  s Values  for  Semi  solid  Amalgam. 

Neumann  Bruce 


G.5  M MnS04 


-.315  volts 


-.300  volts 


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The  solutions  of  manganous  sulphate  and  chloride,  used  in  making 
up  the  half  cells,  were  always  made  up  fresh  without  heating.  This 
was  necessary  because  these  solutions  always  developed  a precipitate 
of  MnO ( OH ) s upon  standing  several  days,  or  immediately  upon  heating, 
except  in  the  case  of  the  very  dilute  solutions  (m  = .0009)  and  in 
the  case  of  a saturated  solution  of  manganous  chloride.  The  latter 
was  toiled  without  obtaining  any  precipitate  and  even  upon  standing 
for  several  weeks  in  a flask  without  a stopper  no  precipitate  was 
noticed. 


IV 


SUMMARY 


1.  Manganese  prepared  by  the  thermit  or  carbon  fusion  process 
cannot  be  amalgamated  at  ordinary  temperatures, 

2 * The  best  method  of  preparing  manganese  amalgam  is  by  elec- 
trolyzing a hot,  concentrated  solution  of  manganous  sulphate,  at 
about  4 volts,  using  a mercury  cathode. 

?.  Manganese  amalgam  ccn tains  a compound  of  manganese  and  mer- 
cury, probably  Mn2Hg5 . 

4 o The  solubility  of  manganese  in  mercury  at  room  temperature 

o 

is  not  greater  than  0.465  /Q . 

5.  The  rate  of  oxidation  of  manganese  in  manganese  amalgam  ex- 
posed to  laboratory  air  is,  roughly,  inversely  proportional  to  the 


15 


per  cent  of  manganese  present.  Solid  amalgam  containing  9.9  /o  man- 
ganese does  not  oxidize,  saturated  amalgams  oxidize  slowly,  while  di- 
lute amalgams  oxidize  instantly. 

„ A 

6.  The  rate  of  oxidation  of  manganese  in  manganese  amalgams  un- 
der water  exposed  to  laboratory  air  is,  roughly,  inversely  propor- 
tional to  the  amount  of  manganese  present. 

7.  The  oxidation  of  manganese  in  manganese  amalgam  by  water  solu- 
tions is  due  to  dissolved  oxygen  and  not  to  decomposition  of  the 
water. 

8.  Manganese  amalgam  does  not  decompose  pure  water  free  from  air. 

9.  The  film  of  oxide  which  forms  on  manganese  amalgam,  when  ex- 
posed to  air,  appears  to  prevent  further  oxidation;  but  the  film  of 
oxide  which  forms  when  the  amalgam  is  in  contact  with  water  does  not 
present  further  oxidation. 

10.  Electrode  potential  measurements  are  given  for  half  cells  of 
bhe  types  Mn  Hg  in  contact  with  z M.Mn  , Mn  02  |z  M.Mn++,  z M.KMnO*, 
Mn08  | ZM.  KMnCh,  z M.KMnCh  in  0.005  MKOH,  z M.KMn04  in  0.05  K.haSO^, 
Mn0a  | z M.KMnO*  in  0.005  M.K0H,  and  Mn02  | z M.KMnO^  in  0.05  N.HgSO^. 

11.  The  electrode  potential  of  dilute  manganese  amalgam  plotted 
against  concentrations  of  manganous  sulphate  gives  a straight  line. 

12.  The  electrode  potential  of  mercury  plotted  against  concen- 
trations of  manganous  sulphate  gives  very  nearly  a straight  line. 

13.  The  electrode  potential  of  semisolid  manganese  amalgam 

o 

(0.465  /°  Mn)  plotted  against  concentrations  of  manganous  sulphate 


16 


gives  very  nearly  a straight  line* 

t 

14.  In  all  combinations  the  electrode  potential  increased  with 
increased  concentration  of  manganese  in  the  amalgam* 

15#  Addition  of  manganese  dioxide  causes  a slight  increase  in 
the  electrode  potential  of  the  half  cells  described. 

16.  The  electrode  potential  of  manganese  amalgam,  in  contact 

with  0.500  molal  manganous  sulphate,  jumps  from  -0.704  volts  for  no 

o . 

manganese  to  0.789  volts  for  1.92  /o  manganese  and  then  remains  con- 
stant, to  within  0.013  volts,  up  to  and  past  the  concentration  at 
which  the  amalgam  becomes  saturated. 

17.  The  manganese  amalgam-manganese  dioxide  electrode  cannot  be 
used  as  a standard  for  measuring  oxidizing  potentials  of  solutions, 
Decause  of  the  ease  with  which  manganese  amalgam  is  oxidized.  Any 
oxidizing  solution  rapidly  oxidizes  the  manganese  thus  decreasing  its 
own  concentration  and  also  that  of  the  amalgam,  which  results  in  a 
lowering  of  the  electrode  potential.  Steady  and  accurate  values  can- 
lot  be  obtained.  The  electrode  is  apparently  irreversible. 

18.  Solutions  of  manganous  sulphate  and.  chloride,  unless  very 
dilute  or  very  concentrated,  always  develop  a precipitate  of  Mn0(0H)2 
ipon  standing  several  days,  or  immediately  upon  heating. 


- 


17 


1. 

2. 

3. 

4. 

5. 
6 • 

7. 

8. 
9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 


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